1. Field of the Invention
The present invention relates to an optical writing device including a photodetector for detecting the quantities of light emitted from a plurality of light-emitting points, and an image forming apparatus provided with the optical writing device.
2. Description of Related Art
In the field of image forming apparatuses, there have been a demand for reducing the size, and the demand is getting stronger. In order to comply with the demand, optical writing devices used in image forming apparatuses are being changed from an optical scanning type to a line optical type. (In the following, an optical writing device will be also referred to as a PH (print head)). An optical writing device of the optical scanning type scans a light beam emitted from a light source (for example, a laser diode) on a photoreceptor surface via a scanning optical system such as a polygon mirror or the like. An optical writing device of the line optical type scans light emitted from a light-emitting-point array including a multiple of very small light-emitting points arranged in a line on a photoreceptor surface directly not via a scanning optical system.
As one of the optical writing devices of the line optical type, there has been developed a device having a light-emitting-point array including a plurality of LEDs (light-emitting diodes) arranged in a line. (Such a device will be hereinafter referred to as an LPH.) With respect to the LPH, generally, the light-emitting-point array and a drive circuit for controlling light emission from the light-emitting-point array are mounted on separate substrates. Accordingly, the manufacturing cost of the LPH becomes higher.
From the background, recently, as a low-cost optical writing device of the line optical type, there has been suggested a device comprising a light-emitting-point array including a plurality of organic LEDs (organic light-emitting diodes) arranged in a line. (Such a device will be hereinafter referred to as an OLED-PH.) With respect to the OLED-PH, the OLEDs and thin-film transistors (TFTs) can be mounted on the same substrate. Accordingly, a low-cost optical writing device can be realized.
OLEDs, however, have light-quantity degradation characteristics as follows: (1) the quantity of light becomes smaller (the light-quantity degradation) as the cumulated light-emitting time increases; (2) the rate of progression of light-quantity degradation differs depending on luminance; and (3) the degree of light-quantity degradation differs depending on temperature.
In the OLED-PH, different OLEDs have different accumulated light-emitting times depending on images to be written, and because of the light-quantity degradation characteristics above, different pixels have different degrees of light-quantity degradation. In order to deal with this problem, the OLED-PH needs to be subjected to light-quantity adjustment in a pixel-by-pixel manner as disclosed by Japanese Patent Laid-Open Publication No. 2007-276355. According to Japanese Patent Laid-Open Publication No. 2007-276355, a light-quantity detection circuit comprises: light-emitting points (OLEDs); a substrate supporting the light-emitting points; a light-quantity detector supported by the substrate, the light-quantity detector located in front of the light-emitting points; and an integrating circuit for integrating outputs from the light-quantity detector. In this structure, even a low-sensitivity photodetector can carry out satisfactory light-quantity detection.
Not only in the OLED-PH but also in any other optical writing device wherein the quantities of light emitted from a plurality of light-emitting points are detected by a single photodetector, the quantities of light incident from different light-emitting points to the light-quantity detector are different. In this case, the photodetector has different dynamic ranges for different light-emitting points. For example, the smaller the quantity of incident light, the lower the dynamic range, and the photodetector has a problem that the detection accuracy of the photodetector for a smaller quantity of light is lower.
The present invention provides an optical writing device with a higher detection accuracy, and an image forming apparatus comprising the optical writing device.